The problem isn't authorizing per-play.
The problem is that each drm'd song gets authorized for each computer you own. So, if you buy a new computer, format your existing one, buy a new ipod, etc. and try to get your backed up songs authorized on the new(or reformatted) device - yep, you need the internet and apple's servers.
So, yeah you keep you have full access to your music after iTunes closes shop. But not *forever* as "buying" implies - unless your current devices are all you will ever use and never break.